1. Field of the Invention
This disclosure relates to an electrolyte for a rechargeable lithium battery and a rechargeable lithium battery including the same.
2. Description of the Related Art
Lithium rechargeable batteries have recently drawn attention as a power source for small portable electronic devices. They use an organic electrolyte solution and as a result have twice the discharge voltage of a conventional battery using an alkaline aqueous solution, and accordingly have a higher energy density.
As for negative active materials of a rechargeable lithium battery, various carbon-based materials such as artificial graphite, natural graphite, and hard carbon, which can all intercalate and deintercalate lithium ions, have been used.
As for positive active materials of a rechargeable lithium battery, chalcogenide compounds that are composite metal oxides such as LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxO2 (0<x<1), LiMnO2, Li[NiCoMn]O2, and the like, are used. For an electrolyte, a lithium salt dissolved in a non-aqueous solvent including ethylene carbonate, dimethyl carbonate, diethyl carbonate, and the like has been used.
During initial charge of a rechargeable lithium battery, lithium ions, which are released from the lithium-transition metal oxide positive electrode of the battery, are transferred to a carbon-based negative electrode where the ions are intercalated into the carbon.
Because of its high reactivity, lithium reacts with the carbon-based negative electrode to produce Li2CO3, LiO, LiOH, etc., thereby forming a thin film on the surface of the negative electrode. This film is referred to as an organic solid electrolyte interface (SEI) film. The organic SEI film formed during the initial charge not only prevents the reaction between lithium ions and the carbon-based negative electrode or other materials during charging and discharging, but it also acts as an ion tunnel, allowing the passage of only lithium ions.
The ion tunnel prevents the disintegration of the structure of the carbon negative electrode, which causes organic solvents in an electrolyte with a high molecular weight to make solvate lithium ions, and the solvent and the solvated lithium ions co-intercalate into the carbon-based negative electrode. Once the organic SEI film is formed, lithium ions do not again react with the carbon electrode or other materials such that the amount of lithium ions is reversibly maintained.
However, problems may occur in which gases are generated inside a battery using carbonate-based organic solvents due to decomposition of a carbonate-based organic solvent during the organic SEI film-forming reaction. These gases include H2, CO, CO2, CH4, C2H6, C3H8, C3H6, etc., depending on the type of non-aqueous organic solvent and negative active material used.
Due to gas generation inside the battery, the battery may expand during charge. In addition, an SEI film is slowly disintegrated by electrochemical energy and heat energy, which increases with the passage of time when the fully charged battery is stored at a high temperature after it is charged, for example, if it is stored at 85° C. for four days after a 100% charge at 4.2 V.
Accordingly, a side reaction in which an exposed surface of the negative electrode reacts with surrounding electrolyte occurs continuously to generate gases. The internal pressure of the battery increases with this generation of gases. Therefore, what is needed is the development of an electrolyte additive that suppresses a volume expansion of the rechargeable lithium battery by preventing or reducing this generation of gases during SEI film-forming reactions.